Radar level gauge (RLG) systems are in wide use for determining the filling level of a product contained in a tank. Radar level gauging is generally performed either by means of non-contact measurement, whereby electromagnetic signals are radiated towards the product contained in the tank, or by means of contact measurement, often referred to as guided wave radar (GWR), whereby electromagnetic signals are guided towards and into the product by a probe acting as a waveguide. The probe is generally arranged to extend vertically from the top towards the bottom of the tank. The probe may also be arranged in a measurement tube, a so-called chamber, that is connected to the outer wall of the tank and is in fluid connection with the inside of the tank.
The transmitted electromagnetic signals are reflected at the surface of the product, and the reflected signals are received by a receiver or transceiver comprised in the radar level gauge system. Based on the transmitted and reflected signals, the distance to the surface of the product can be determined.
More particularly, the distance to the surface of the product is generally determined based on the time between transmission of an electromagnetic signal and reception of the reflection thereof in the interface between the atmosphere in the tank and the product contained therein. In order to determine the actual filling level of the product, the distance from a reference position to the surface is determined based on the above-mentioned time (the so-called time-of-flight) and the propagation velocity of the electromagnetic signals.
Most radar level gauge systems on the market today are either so-called pulsed radar level gauge systems that determine the distance to the surface of the product contained in the tank based on the difference in time between transmission of a pulse and reception of its reflection at the surface of the product, or systems that determine the distance to the surface based on the phase difference between a transmitted frequency-modulated signal and its reflection at the surface. The latter type of systems are generally referred to as being of the FMCW (Frequency Modulated Continuous Wave) type.
In any case, the propagated electromagnetic signal is typically not only reflected at the impedance transition constituted by the interface between atmosphere and surface, but at several other impedance transitions encountered by the signal. Such impedance transitions may, for example, result from fixed structures in the tank or, in the case of a GWR-system, product residue that may have adhered to the probe as the filling level of the product changes inside the tank.
There is therefore a risk that the system attempts to determine the filling level based on the wrong reflected signal. This is especially the case when the product inside the tank has similar signal propagation characteristics as the atmosphere in the tank. This results in a small impedance transition and, accordingly, a relatively weak echo signal. Examples of products yielding relatively weak echo signals are liquid natural gas (LNG), liquid petroleum gas (LPG), oil-based products, solids such as plastic pellets or grain etc.
One way of reducing the risk for such an erroneous determination of the filling level is to make a reference filling level measurement when the tank is empty. A typical result from such a reference filling level measurement is a disturbance echo profile, in which echoes representing disturbances that may be present in the tank are visible. This disturbance echo profile can be used to modify an echo profile obtained during normal measurement conditions. For example, for a pulsed RLG system, at least a part of the disturbance echo profile may be subtracted from the echo profile obtained during normal measurement conditions.
However, conditions in the tank generally vary over time in such a way that existing disturbances move and/or new sources of disturbance echoes are added, such as, for example, lumps of material, such as oil, adhering to the probe in the case of GWR-measurements.
Such varying conditions may be taken into account by determining a disturbance echo profile during normal measurement conditions, when the tank is not empty. However, such a disturbance echo profile can then only be reliably determined above the surface of the product contained in the tank, because of the relatively stable propagation characteristics in the atmosphere above the surface of the product.
U.S. Pat. No. 6,078,280 discloses a method for determining a disturbance echo profile above the surface of a product contained in a tank, involving automatically determining a transition point that defines which part of a newly acquired echo profile to use for updating a previously stored disturbance echo profile. According to U.S. Pat. No. 6,078,280, this transition point is determined based on the surface echo signal.
Under certain conditions, it may, however, be difficult to determine the surface echo signal, which may result in that the surface echo signal is included in the disturbance echo profile when the method disclosed in U.S. Pat. No. 6,078,280 is used, which may lead to an incorrect filling level determination.